Polyesters from beta-hydroxyalkyl diesters of sulfonyl dibenzoic acid condensed with a dihydroxy compound



POLYESTERS FROM fi-HYDROXYALKYL DIESTERS F SULFONYL DIBENZO-IC ACIDCONDENSED WITH A DIHYDROXY COMPOUND John R. Caldwell, Kingsport, Tenn.,assignor to Eastman Kodak Company, Rochester, N. Y., a corporation ofNew Jersey No Drawing. Application October 3, 1952, Serial No. 313,070

11 Claims. (Cl. 260-75) 7, This application relates to valuable linearpolyesters prepared by condensing a B-hydroxyalkyl diester of p,p'-sul'fonyl dibenzoic acid with one or more of various dihydroxy compoundsdescribed herein below.

It is an object of this inventionto provide a process as describedherein for preparing valuable linear polyesters. Other objects willbecome apparent hereinafter.

This application is a continuation-in-part of my copending application,Serial No. 143,594, filed February 10, 1950, now United States PatentNo. 2,614,120 dated October 14', 1952. In that application sulfonyldibenzoic acid is called bis ('dicarboxy diphenyl sulfone). Thepolyester products d'escribed'herein are also described and claimed inmy copending applications filed on even date herewith, Serial No.313,061 and Serial No. 313,067.

Highly polymeric esters of terephthalic acid and various glycols arewell known and have been used in the preparation of linear, highlypolymeric polyesters having properties including that of being capableof being formed into useful filaments, fibers and the like, and havinghigh melting points anda lowdegree of solubility in organic solvents.

It has now been found that the ti-hydroxyalkyl diesters (especially the,B-hydroxyethyl diester) of p,p'-su1fonyl dibenzoic acid can becondensed with various dihydroxy compounds (as described below) toproduce a new kind United Paw- 0 of linear polyester having highlyvaluable properties temperatures can be prepared from these newpolyesters.

These fibers, films, etc. have exceptionally high tensile strength andelasticity. The fibers have softening points well above 160 C. (oftenabove 200 C.) and show excellent resistance to most organic solventseven at ele- My new polyesters can. be extruded vated. temperatures. inthe form of films or sheets which are. especially valuable asphotographic film. base material because of their excellent dimensional.stability and. resistance to swelling by water. These properties arefurther described in my copending applications referred to above.

The novel. process employed in accordance with this invention is basedon the fact that. the fi-hydroxalkyl diesters of p,p'-sulfonyl dibenzoicacid can. be readily pre pared as described in my parent application(Serial No. M3594) and are especially valuable inasmuch as I havediscovered that they can be readily converted into new and useful linearpolyesters by heating in the presence of the dihydroxy compoundsdescribed in greater detail below. Although it is possible to employ thefree p,p'- sulfonyl dibenzoio acid in the preparation of polyesters, thehigh degree of insolubility thereof and its slow rate of reaction, makeit much more. advantageous. to convert the: free acid: into a. diester'thereof: prior to the preparation of linear polyesters therefrom. Thealkyl diesters can be prepared: by refluxing the free acid with a large"ice excess of an alkyl alcohol. Standard esterification catalysts andprocedures can be employed; however, when the known catalysts of theprior art are employed in the preparation of linear polyesters fromthese alkyldiesters, the results have not been found by myinvestigations to possess the unexpectedly high degree of utility thatexists when the fi-hydroxyalkyl diesters are employed. I

A process for preparing these fl-hydroxyalkyl diesters is disclosed inmy parent application whereby an alkylene oxide is reacted with the freeacid in the presence of a tertiary amine catalyst. The fi-hydroxyethyldiester is the diester of principal importance in preparing linearpolyesters since the other -B-hydroxyalkyl diesters tend to result inlower melting polyesters, especially when the diester is self-condensedor is condensed with a small proportion of additional glycol. When largeproportions of additional glycol of the types described below areemployed it becomes less important to exclude the other B-hydroxyalkyldiesters (particularly the fl-hydroxypropyl diester) from the process ofthis invention since branched-chain alkylene linkages will be present inthe polyester chain to a relatively small extent.

The polyesterproducts prepared from the fl-hydroxypropyl diestercondensed with itself or with itself. in admixture with other glycols.or in the presence of a stoich-iometrical excess of a. glycol or glycolsgive products which have relatively wide softening ranges and good flowcharacteristics whereby they are quite valuable for the preparation ofshaped objects by molding. or extrusion methods. In accordance with myinvention polyesters can also be prepared employing the fl-hydroxyalkyldiesters in condensation with aliphatic ether glycols whereby the.products have a relatively wide softening range and good flowcharacteristics which are quite useful. in molding, or extrusionoperations. The softening range and flow characteristics are improved byemploying mixtures. of aliphatic ether glycols with other aliphaticether glycols or polymethylene glycols.

These novel linear polyesters can be prepared so as tosoften at thetemperatures which are about l -200 or higher. Useful linear polyesterscan also be prepared which soften at somewhat lower temperatures. Thepolyesters having melting or softening temperatures in the lower part ofthe overall range applicable to these polyesters are useful in themanufacture of electrical insulatioil, gaskets, packing, flexibletubing, rods, sheets, etc.

When the m,m and the m,p'-isomers of p,p-'sulfonyl dibenzoic aciddiesters are employed in admixture with the p,p-isomer, the linearpolyesters product has a lower melting point and softening temperature.This same elfect is produced when homologs of the p,p"isomer and theother isomers are incorporated into the linear polyester. I-f homologsare employed they are most advantageously those of the p,p-isomer, e. g.m-,m' dimethylp,p*-su-lfonyl dibenzoic acid, o-e'thyl-p,p-sulfonyldibenzoic acid, m-methyl-o-propyl-p,p-sulfonyl dibenzoicacid, etc. Smallproportions of the fi-hydroxye'thyl diester of such isomers and homologscan be employed in substitution for a corresponding quantity ofp,p"'-sulfonyl dibenzoic acid diester when the linear polyester productis not intended to be used in the preparation of fibers. In fact, whenthe polyester is to be employed for purposes other thanv making. fibers,substantial quantities of these isomers and homologs can be employed. ifan. exceptionally highmelting point (e. g.,over 20th C.) is notnecessarily desired but a wide softening temperature range is deemed.advantageous. Moreover, when thesole dirlrydroxy compound employed'is apol-methylene glycol, useful polyesters can: be. prepared employing theB-h'y- 3 droxyalkyl diesters of either the m,m'- or m,p'-isomers (seeexamples 4 and 5 below).

Two of the outstanding qualities of the polyesters of this invention aretheir excellent dimensional stability and low degree of waterabsorptivity. This results in superior resistance to dimensional changedespite changes in atmospheric humidity or immersion in aqueoussolutions. These qualities result in this linear polyester being wellsuited to the manufacture of photographic film base.

According to one embodiment of my invention, a process is provided forpreparing a linear polyester comprising (A) condensing an acid diesterhaving the following general formula:

earlier part of the condensation is from about 150 C. to about 220 C.However, higher and lower temperatures can also be employed. The uppertemperature depends upon the boiling point of whatever glycol or glycolsmay be employed. Advantageously, the low pressure defined under (F) isless than about mm. of Hg pressure. Most advantageously the pressure isless than about 5 mm. of Hg pressure. Most advantageously the dioxycompounds are aliphatic ether glycols and polymethylenc glycols havingformulas as depicted above wherein R5 and Rs represent hydrogen atoms.There is a gradual modification of properties produced by employingaliphatic ether glycols in conjunction with polymethylene glycols R1wherein R1 and R4 each represents a member of the group consisting of ahydrogen atom and a methyl radical, (B) with a dioxy compound selectedfrom the group consisting in increasing proportions. Advantageously thealiphatic ether glycol is used to the extent of 30 mole percent or moreof the combined dioxy compounds employed when of (1) a dihydroxy sulfonediester having the following it is desired to obtain polyesters whichclearly exhibit general formula:

properties contributed by the aliphatic ether glycols.

CH3 (2) an aliphatic ether glycol having the following formula: R50(R7-OR'1ORs wherein R7 represents an alkylene radical containing from 2 to 4carbon atoms and q represents a positive integer of from 1 to 10, R5 andRs each represents a substituent selected from the group consisting of ahydrogen atom and an acyl radical containing from 2 to 4 carbon atomsand (3) a polymethylene glycol having the following formula:

wherein 1 represents a positive integer of from 5 to 12, the dioxycompound being employed in such a proportion that there is at least anequivalent amount of oxy substituents in proportion to the carboxysubstituents in the overall combination of the diesters and the dioxycompounds, (C) in the presence of a condensing agent selected from thegroup consisting of the alkali metals, the alkaline earth metals, theoxides of these two groups of metals. the alkoxides containing from 1 to8 carbon atoms of these two groups of metals, the carbonates and boratesof these two groups of metals, lead oxide (litharge), and compoundshaving the following formulas:

wherein M represents an alkali metal, M represents an alkaline earthmetal selected from the group consisting of magnesium, calcium andstrontium, R represents an alkyl group containing from 1 to 6 carbonatoms, R, R" and R' each represents a member of the group consisting ofR and an aryl group of the benzene series containing from 6 to 9 carbonatoms and Hal represents a halogen atom, (D) at an elevated temperature,(E) the condensation being conducted in an inert atmosphere, and (F) thelatter part of the condensation being conducted at a very low pressureof the inert atmosphere.

When the dioxy compound (or compounds) is an aliphatic ether glycol or apolymethylene glycol, it is advantageously employed in such a proportionthat there are from about 1.2 to about 3 oxy substituents in proportionto the carboxy substrtuents in the overall combination of the aciddiester and the dioxy compound. Advantageously, the elevated temperatureemployed during the The glycols defined above may not actually containany free hydroxy radicals since they may be in esterified form asindicated by the formulas given. However, these hydroxy or substitutedhydroxy radicals are referred to generically as oxy radicals orsubstituents. The dioxy compounds which can be employed in accordancewith this invention are most advantageously dihydroxy compounds; suchcompounds will hereinafter be referred to as dihyclroxy compoundsalthough it is to be understood that dioxy compounds of the typedescribed above are intended to be covered by this term. Similarly, thesulfonyl dibenzoic acid diesters defined above do not actually containany free carboxy radicals since they are in completely esterified form;however, the

COO-C-U- -(H:OTI

1 h nr t radicals are generically referred to as carboxy radicals in thedescription of the above process.

in the description of the process given above, the compound under (A) iscalled an acid diester which is the same in one instance as the first ofthe di'nydroxy compounds described under (B) which is called a dihydroxysnlfone diester. This distinction in terminology is cm ployed forconvenience in maintaining a distinction between the two reactantsemployed to form the polyesters. If the acid diester is afi-hydroxyethyl diester and if it were condensed with itself, a linearpolyester would be produced which melts at such a high temperature thatit would decompose before a sufficiently high molecular weight could beobtained (see my copending application filed on even date herewith,Serial No. 313,061). It '1', therefore advantageous to condense the aciddiester defined under (A) with itself only when it is the,G-hydroxypropyl diester whereby a polyester can be obtained which meltsat about 180 C.

When the fi-hydroxyethyl diester is employed, the polymethylene glycolis restricted to those which contain from 5 to 12 carbon atoms unless asubstantial quantity of one of the other dihydroxy compounds is presentin order to reduce the melting point below that at which decompositiontakes place, in which event a polymethylene glycol containing from 2 to12 carbon atoms can be employed. This is discussed in greater detail inmy copending application filed on even date herewith, Serial No.313,061.

The products of this invention are linear polyesters having meltingpoints on the order of 180 200". C. (generally at least C.) which arebelow the decomposition temperature of the; polyester, which polyesterseontainthe following repeating unit:

wherein. R1 represents a hydro en. atom or a. methyl radical. It R1.represents a. hydrogen atom, these linear polyesters. also. contain asubstantial. proportion of either or both the followingv repeatingunits:

wherein p, q and R7 are defined above. These polyesters are capable ofbeing formed into fibers (such as by melt spinning methods) especiallywhen R1 represents an ethylene radical, q is 1, and p isa positiveinteger of from 2 to 8. These. fibers: are capable of. being: to from 3;to 6; times their originally spun lengthqwhereby these fihersdevelopstrong, elastic and. highly valuable properties. a

. Examples of the aliphatic ether glycols which can.- be employedadvan-tageously in accordance. with. this invention include diethyleneglycol, triethylene glycol, tetraethylene glycol, bis 4-hydroxybutyl),ether, and his (3-hydroxypropyl') ether; thus q in the. formula setforth above is advantageously from 1 to 4 (most advantageously q is, 1).Qther suitable aliphatic ether glycols. having the following formulascan be employed in. addition to. those mentioned above:

The ether linkages can be separated by branched chain alkyl radicals asindicated in the above formulas; however, higher melting polyesters areproduced when the var ous atoms in the aliphatic ether glycol constitutea straight chain. When branched chain aliphatic. ether glycols areemployed, it is advantageous to combine such a glycol with a substantialproportion of polymethylene glycol in order to enhance the melting and"softening temperatures of the polyester product. As indicatedabove, monoor diesters of these aliphatic ether glycols can also beemployed. Thus,the acetates, propionates and butyratesare examples of' such esters.Mixtures of 2 or more of these aliphatic ether glycols can also beemployed. 7

Examples of the polymethylene glycols which can be employed inconjunction with thealiphatic ether glycols in accordance with thisprocess of this invention include ethylene, trimethylene,pentamethylene, decamethylene and other such glycols. Note that undercertain circumstances only glycols having at least carbon atoms can beemployed. As indicated above, mono or die'sters of these glycols canalsov be employed. Thus, the acetates, propionates and butyrates areexamples of suchesters. Mixtures' of two or more of these polymethyleneglycols can also be employed in conjunction with one or more of thealiphatic ether glycols. Generally, the more components there are in thedihydroxy composition, the greater will be the tendency towardproducing; a lower melting polyester product; accordingly; itisgenerally most advantageous to employ no more than one aliphatic etherglycol and only one polymethylene glycolin pr'epar ing any givenpolyester.

The. catalytic condensing, ageutswhich, can be cmpl yfid have beendescribed above. From about 0.005% to about perature is referred to inseveral instances.

lower percentages can also be employed. Generally,

firomabout 0.01. to about 0.06% of the catalytic condensing agent can beadvantageously employed based on the weight of the sulfionyl dibenzoicacid diesters. being condensed.

The temperature at which polyesterification can be conducted isdependent upon the specific reactants involved in any given reaction. Ingeneral, the reaction mixture can be heated at from about to about 220C. for from approximately two to three hours in an inert atmosphere (e.g. nitrogen or hydrogen); the mixture can then be heated at from about225 -240 to about 2:8.0"-3 l0 C. in the same atmosphere forapproximately one to two hours. Finally, the pressure can be greatlyreduced toform a vacuum (less than about 15 mm. of Hg pressure butpreferably on the order of less than 5 mm. of Hg pressure) while thetemperature is maintained in the same range (225 -310 (3.); theseconditions are advantageously maintained for approximately four to sixadditional hours. The conditions can be varied considerably dependingupon the degree of polyesterification desired, the ultimate propertiessought, the stability of the. polyester being produced and the use forwhich the productis intended. I

The reaction can be carried out in the presence or absence of a solvent.Inert, high boiling compounds, such as diphenyl ether, diphenyl, mixedtolyl sulfones, chlorinated. naphthalene, chlorinated diphenyl, dimethylsulfolane, etc. can be used as the reaction medium.

It has been found that the type of catalyst used has an importantbearing upon the properties of the final product. Although most of thecatalysts cited in the prior art can be used, it has been found thatcertain novel catalysts give superior results. The aluminum andtitanium. all'coxid'e complexes described in copending applicationsfiled on even date herewith are especially valuable for the preparationof the polyesters described here. Moreover, novel tin catalysts havealso been found to be especially eifecti-ve. See Caldwell Serial No.313,072, Serial No. 313,078, Caldwell & Reynolds Serial No. 313,077,Wellman 86. Caldwell Serial No. 313,074, Serial No. 313,075 and SerialNo. 313,076, and Wellman Serial No. 313,073 for a description ofespecially advantageous catalytic condensing agents.

It is important to' exclude oxygen and moisture at all stages of thecondensation reaction. Inert atmospheres which can be advantageouslyemployed include nitrogen, hydrogen, helium, etc. During the. processgood agitation is provided. Substantially anhydrous reactants can alsobe advantageously employed although this is not essential especially ifany Water is removed in the earlier stages of the condensation.

In. the examples given below, thev hot bar sticking tem- The hot barsticking test-can be briefly described as follows. The polyester fiberis placed onthe flat surface of a heated bar and a weight of I00 g. isapplied to the fiber along a distance of inch. of the fiber length. Thecontact surface of this weight has a' coating, ofpolytetrafluoroethylene which acts as a thermal insulator. The fiber isallowed to remain in contact with the bar under this weight for oneminute. The minimum temperature at which the fiber adheres to the hotbar under these conditions. is the sticking temperature as that term isemployed in. the. examples given herein.

This invention can be fiurther illustrated by the following examples; inadditionflto these examples. it. is apparent that other variations andmodifications thereof can be adaptedto obtain similar results. .ExampleI.-/8-hydr0xyethyl ester and diethylene glycol Three hundredv andninety-four. grams. 1.0 mol) of the. fl-hydroxyethyl ester ofp,P'-sulfony1di benzoic acid and 212 g. (2.0 mols) diethylene glycolwere placed in a reaction vessel equipped with a stirrer and inlet forpurified nitrogen. Magnesium chips, about 0.2 g. in weight, were heatedat 100l20 C. with iodine crystals to activate the surface, and added tothe reaction mixture. The mixture was heated and stirred at 190-200 C.in an atmosphere of purified nitrogen for 30 minutes. The temperaturewas then raised to 270280 C. and held for 1 hour to distill 01f excessethylene and diethylene glycols. A vacuum of 1.0 mm. was then applied,and stirring was continued for 4 to 5 hours. The product obtained has aninherent viscosity of 0.60 in a mixture of 60% phenol- 40%tetrachlorethane. It softens at ISO-200 C. The product is useful for thepreparation of molded objects. It can be extruded in the form of rods,sheets, films, and tubes. The polymer contains essentially 50 molpercent each of ethylene glycol and diethylene glycol.

Example II.--,3-hydroxyethyl diester and decametlzylene glycol Threehundred and ninety-four grams (1.0 mol) of the fi-hydroxyethyl ester ofp,p'-sulfonyldibenzoic acid and 261 g. (1.5 mol) of decamethylene glycolwere placed in a reaction vessel equipped with a stirrer, a distillationcolumn, and an inlet for purified hydrogen. Magnesium chips (0.2 g.)were heated with iodine to activate the surface and added to the vessel.A solution of 0.1 g. lithium in cc. ethyl alcohol was added. The mixturewas stirred at 180-l90 C. for 30 minutes in an atmosphere of purifiedhydrogen and the temperature was then raised to. 250-260 C. Thedistillation column was adjusted so that ethylene glycol was removed butwith the decamethylene glycol being returned to the reaction mixture.After substantially all of the ethylene glycol had been distilled, avacuum of 1-2 mm. was applied. Stirring was continued for 56 hours. Apolyester having an inherent viscosity of 0.7 in 60 phenol-40tetrachlorethane was obtained. This product is substantially puredecamethylene glycol polymer and contains very little ethylene glycol.The polyester melts at about 200 C. It is useful for the manufacture offibers, films, and molded objects.

Example Ill.;3-hydr0xyethyl dicstcr and octamethylene glycol Theprocedure described in Example II was repeated exactly except that anequimolecular quantity of octamethylene glycol was employed in lieu ofthe 1.5 mol of decamethylene glycol, there being a suitable adjustmentin the distillation operation to retain the octameth- I ylene glycolwhile distilling off ethylene glycol. The polyester obtained wassubstantially a pure polyester containing only octamethylene radicals inthe repeating units. This polyester melts at about 210215 C.

Example I V 3-hydr0.ryethyl diester condensed with same diestcr ofm,m'-is0mer of the acid Two mols of m,m'-sulfonyldibenzoic acidfl-hydroxyethyl ester and 1 mol of p,p-sulfonyldibenzoic acid ,6-hydroxyethyl ester were heated with magnesium catalyst, 0.2 gram,activated by heating with iodine. The equipment employed was the same asin Example I. An inert polyester obtained is useful as a molding plasticand for preparing films.

Example V.-B-hydr0xyethyl diester of m,p'-isomer of acid self-condensedThree hundred and ninety-four grams (1.0 mol) of m,p-sulfonyldibenzoicacid fl-hydroxyethyl ester were placed in a reaction vessel equippedwith a stirrer. Magnesium turnings (0.2 g.) activated by heating withiodine were added as catalyst. The vessel was heated at 230--230 C. andthe stirrer was started. An inert atmosphere of purified nitrogen wasemployed. Ethylene glycol began to distill as the ester interchange tookplace. The temperature was raised to 260-270 C. during 2 hours and heldat this temperature for 1 hour. A vacuum of 1 mm. was applied for 5hours. The product obtained is useful as a molding plastic.

Example V I .-;3-hydroxyethyl diester and hexamethylcnc glycol Theprocedure described in Examples II and III was repeated exactly exceptthat an equimolecular quantity of hexamethylene glycol was employed. Thepolyester obtained melts at about 220 C. and is useful in preparingfibers, films, etc.

Example VII.-B-hydroxyethyl diester, diethylene glycol andtetramethylcne glycol The procedure described in Example I was repeatedexactly except that half of the 2 mols diethylene glycol was replacedwith 1 mol of tetramethylene glycol. The polyester product obtainedcontains repeating units consisting of approximately 50% (mole basis) ofunits containing the ethylene radical, 25% containing the diethyleneradical and 25% containing the tetramethylene radical. This polyesterhad properties similar to the product obtained in Example I except for ahigher softening temperature. This product can be used to form fibers,tubes, films, etc.

Example VIII.-;3-hydroxyethyl diester, tetraethylene glycol andtrimethylene glycol The procedure described in Example I was repeatedexactly except that in lieu of the 2 mols of diethylene glycol there wasemployed 1 mole of tetraethylene glycol and 1 mole of trimethyleneglycol. The polyester obtained has properties similar to those describedfor the product of Example I. The product has a relatively widesoftening range and is useful in extruding and molding shaped objects,e. g. tubes, rods, sheets, balls, etc. The polymer contains repeatingunits containing ethylene, tetraethylene and trimethylene radicals.

The procedure described in Examples II, III and VI can be employed toprepare polyesters from aliphatic ether glycols which contain nosubstantial amount of ethylene radicals in the repeating units.

In addition to the catalysts employed in the above examples, other ofthe described catalysts can also be employed, e. g. Ca, Li2CO3,K2BOa,and litharge.

What I claim is:

l. A process for preparing a linear polyester comprising (A) condensingan acid diester having the following general formula:

atmosphere of purified nitrogen was employed. The reaction mixture washeated at about 225 C. with stirring while ethylene glycol distilledoff. The temperature was then raised to 260275 C. over a period of twohours wherein R1 and R4 each represents a member of the group consistingof a hydrogen atom and a methyl radical, (B) with a dioxy compoundselected from the group consisting of (l) a dihydroxy sulfone diesterhavand held in this range for another hour. The pressure ing thefollowing general formula:

was then reduced to about 0.8-1.0 mm. of Hg pressure and heating wascontinued for five more hours. The

CH: (2) an aliphatic ether glycol having the following formula:R5O(R'1-O --R'zORe wherein R1 represents an alkylene radical containingfrom 2 to 4 carbon atoms and q represents a positive integer. of from 1to 10, and R5 an] R6 each represents a substituent selected from thegroup consisting of a hydrogen atom and an acyl radical containing from2 to 4 carbon atoms and (3) a polymethylene glycol having the followingformula:

wherein p represents a positive integer of from 5 to 12, the dioxycompound being employed in such a proportion that there is at least anequivalent amount of oxy substituents in proportion to the carboxysubstituents in the overall combination of the diesters and the dioxycompounds, (C) in the presence of a condensing agent selected from thegroup consisting of the alkali metals, the alkaline earth metals, theoxides of these two groups of metals, the alkoxides containing from 1 to8 carbon atoms of these two groups of metals, the carbonates and boratesof these two groups of metals, and litharge, (D) at an elevatedtemperature, (E) the condensation being conducted in an inertatmosphere, and (F) the latter part of the condensation being conductedat a very low pressure of the inert atmosphere.

2. A process as defined in claim 1 wherein the elevated temperature isincreased gradually during the course of the condensation up to atemperature of from about 225 to about 310 C.

3. A process as defined in claim 2 wherein the condensing agent isemployed inan amount of from about 0.005% to about 0.2% based on theweight of the diesters employed.

4. A process as defined in claim 3 wherein the elevated temperatureemployed during the earlier part of the condensation is from about 150to about 220 C. and the low pressure defined under (F) is less thanabout 15 mm. of Hg pressure.

5. A process as defined in claim 4 wherein all materials employed in theprocess are substantially anhydrous.

6. A process as defined in claim 5 wherein the dioxy compound isemployed in such a proportion when it is a glycol that there are fromabout 1.2 to about 3 oxy substituents in proportion to the carboxysubstituents in the overall combination of the acid diester and theglycol.

7. A process as defined in claim 6 wherein the acid diester is theQ-hydroxyethyl diester of p,p'-sulfonyl dibenzoic acid and the dioxycompound is diethylene glycol.

8. A process as defined in claim 6 wherein the acid diester is thefi-hydroxyethyl diester of p,p-sulfonyl dibenzoic acid and the dioxycompound is decamethylene glycol.

9. A process as defined in claim 6 wherein the acid diester is theB-hydroxyethyl diester of p,p-sulfonyl dibenzoic acid and the dioxycompound is octamethylene glycol.

10. A process as defined in claim 6 wherein the acid diester is thefi-hydroxyethyl diester of p,p-sulfonyl dibenzoic acid and the dioxycompound is hexamethylene glycol.

11. A process as defined in claim 6 wherein the acid diester is the,B-hydroxyethyl diester of p,p'-sulfonyl dibenzoic acid and the dioxycompound is an equimolecular mixture of diethylene glycol andtetramethylene glycol.

References Cited in the file of this patent UNITED STATES PATENTS2,465,319 Whinfield et a1. Mar. 22, 1949 2,547,113 Drewitt et al. Apr.3, 1951 FOREIGN PATENTS 621,977 Great Britain Apr. 25, 1949 650,358Great Britain Feb. 21, 1951

1. A PROCESS FOR PREPARING A LINEAR POLYESTER COMPRISING (A) CONDENSINGAN ACID DIESTER HAVING THE FOLLOWING GENERAL FORMULA